Electrophotographic process



Nov. 15, 1966 r J. w. WEIGL ETAL 2 3 ELECTROPHOTOGRAPHIG PROCESS Filed Oct. 25. 1961 2 Sheets-Sheet I I u FIG. I

II/lll!III/l INVENTORS JOHN W. WEIGL ALAN AM IDON B W M ATTORNEYS 2 Shee'cs-Shee'r.` 2

Filed Oct. 25. 1961 hm mm INVENTORS JOHN W. wElGL ALAN AMIDON ATTORNEYS wN mm United States Patent Filed Oct. 25, 1961, Ser. No. 147538 8 Claims. (CI. 96-1) This invention relates to improvements in electrophotography and, more particularly, to a method and apparatus for establishing electrical conductivity on a surface of photoconductive insulating layers, such as are conmonly used in electrostatic electrophotography.

In conventional electrostatic electrophotography use is made of photoconductive layer, such as a dispersion of photoconductive Zinc oxide in an insulating resinous binder, coated over a conductive base, such as, for eX- ample, aluminum foil. The electrostatic latent images are then formed by first applying a blanket charge to the face or receptive surface of the photoconductive layer while the hacking is electrically grounded. Thereafter, the face of the photoconductive layer is illuminated with an actinic radiation pattern which causes the surface charge to leak from the free surface of the photoconductive layer towards the conductive base.

Electrostatic latent images formed by the patternwise illumination of charged photoconductive layers are generally rendered visible `by the application thereto of particles of electrostatically charged powder which may be applied as dust clouds, as aerosols, as suspensoids in insulating liquids, or as adsorbates on magnetic particles or electroscopic carrier heads. Thereafter, the powder irnages may be transferred to a suitable receiving sheet or afxed directly to the photoconductive layer by wellknown means.

The reason for using a grounded conductive backing is to enable an equivalent image charge, of a polarity opposite to that applied to the free surface of the photoconductive layer, to distribute itself uniformly on the backed surface. This causes the sheet to bear a symmetrical dpole charge rather than a unipolar charge. The image charge layer on the hacking causes the actinically sensitive charge layer on the free face of the photoconductor to be distributed densely and uniformly, and to remain stably attached until discharged by subsequent actinic radiation.

It is believed that the use of a conductive hacking for the photoconductive layer enhances maximal image resolution by permitting lateral motion of image charge through the sheet during latent image formation. The surface charges deposited on the receptive surface are relatively far apart, the mean separation being in the order of 0.1 micron; while at more common surface charge density levels, means separations of 0.2 to 0.3 micron are to be expected. Since charges deposited on an insulating surface tend to be trapped in clusters of 10 to 103 electrons, the actualseparation of surface charges becomes comparable to the thickness of the photoconductor, typically 2 to 10 microns, across which the surface charges face their opposite image charges in the conductive base. Thus, if the image charge is not free to move within the hacking layer, the free surface charge that is liberated by illumination will tend to migrate laterally towards the nearest clusters of image charge, and, as a result, degrade the image resolution. However, whatever the reason or theory for the eifect, it is noted that images of a substantially higher resolution are obtained if a conductive base is substituted for an insulating base behind a given photoconductive layer.

The selection of proper conductive hacking materials is quite difficult. Attempts have also been made to render ice paper adequately conductive, including the use of various humectants and anti-static agents. However, these fail when the ambient humidity drops below about 15% relative humidity, which is the relative humidity at which one frequently must operate in this field. There have been attempts to solve this disadvantage by spraying upon the insulating hacking a corona discharge of a polarity opposte to that used for the free surface of the photoconductor. This technique, as well as others, are inadequate because they do not provide for the lateral conductivity which is required for high resolution image formation, as described above. Aluminum foil, laminated paper and carbon black filled paper, while providing adequate conductive bases are not only difficult .to handle, but are also costly on a commercial scale.

A number.of conductive precoats have also been used in the art. However, such coatings are dfficult to apply and, being opaque to actinic radiation, cannot be used as electrophotographic masters for the preparation of contact copies on diazotype and similar reproduction machines.

It is therefore, a primary object of this invention to provide an improved electrophotographic process which eliminates the need of conductive backings, interlayers, or precoats for the electrophotographic material.

It is a particular feature of the invention that by virtue of the above proposed process, it becomes feasible to use simple coatings of photoconductive insulators on translucent backing material such as insulating papers or film bases having suflicient translucency to serve as diazotype masters.

A further object o-f the present invention is to provide an improved electrographic process whereby only a portion of the photoconductive insulating layer is rendered semi-conductive during processing.

Essentially, the invention embraces the conditioning of photoconductive insulating materials without conductive hacking, to acquire during normal applications thereof in electrostatic electrophotography, a state of suflicient electrical conductivity on a selected surface, thereby exhibiting r the properties of materials having a conductive base. The conditioning is accomplished by pre-illuminating the back surface of the photoconductive layer with actinic radiation prior to the electrostatic charging thereof.

Another object of the invention is to provide a novel combnation of illuminating and charging means whereby the process of the invention may be carried out.

Other objects and features will be apparent from the following description of the invention, pointed out in partcularity in the appended claims, and taken in connection with the accompanying drawings, wherein:

FIGURES 1, 2 and 3 are enlarged schematic crosssectional views of a typical electrophotographic sheet, and

FIGURE 4 is a schematic diagrammatic View of one from of an apparatus designed to carry out the process.

In the preferred form of the invention the back surface of a photoconductive layer is pre-illuminated prior to applying to the front surface thereof an electrostatic charge. It is to be understood that in the material used the support of the photoconductive layer must be translucent or transparent, to the actinic radiation used for the photoconductor in question.

The process in accordance with the invention is also applicabl-e to films -of photoconductive material, which are not coated on the support, but are cast in a form of a film of photoconductive substance, the pre-illumination being applied in this case, to the side opposite to that used for the charging device.

It is belived that this pre-illumination step causes a uniform, temporarily -semi-conductive layer to be formed within the photoconductive layer. Thus, this semi-conductive layer serves the same purpose as the conductive base, and eliminates the separate coating heretofore employed in conventional processes. Moreover, this semiconductive layer may be connected, as required, to ground or to other convenient potential by means of suitable contaet devices.

The process of the present invention is also useful in various forms of Xerography in which a reusable photoconductive layer is normally backed by `a separate translucent or opaque member. Thus, according to the present invention, an external conductive backing may be eliminated 'by pre-illuminating the back of the photoconductor through the translucent member prior to the application of a surface charge.

The back illumination of electrophotographic sheets having photoconductive layers coated upon translucent but poorly conductive backings, yield the following practical advantages. Image density, especially in extended solid areas, is made more uniform, background density is greatly decreased, and image resolution and contrast are thereby improved. Furthermore, the pre-illumination process of this invention eliminates the undesirable effect which is often observed in using liquid immersion development with coatings on poorly conductive bases, namely, a reversed-image deposition of toner on the back side of the sheet.

The pre-illumination used in accordance with this invention is preferably of such wavelengths that only a thin semi-conductive layer is caused to form at the back side of the photoconductor.

It is preferred moreover, that depending on the coating thickness, not more than of the actinic radiation penetrate 'beyond about the first 10% of the layer thickness. This is easily accomplished, since the most practical photoconductors absorb light very strongly in their intrinsic absorbance regions. For example, the nearultraviolet radiation (3200 to 3900 A.U.) provided by a commercial black light fluorescent lamp may be absorbed to an extent of more than 90% by a 0.3 micronthick layer of photoconductive zine oxide. The thickness of useful Zinc oxide electrophotographic layers is usually in excess of about 6` microns. Like results may be o'btained by the use of other oommonly used photocondu-ctors, provided they are pre-illuminated in their ntrinsic regions.

Examples of photoconductors which may be employed, and their intrinsic absorbance region is given in the following tabulation:

' Wavelength region of in- Photoconductor: tense absorption, A.U. Cadmium sulfide 3000-5000 Zinc oxide 2000-3900 Cadmium selenide 4000-9000 Lead sulfide 4000-9000 Silver chloride 2000-2700 Germanium 3000-7500 Silicon 3000-5500 Iso-violanthr-one 7000-1l,000 Lead selenide 5000-12,000 Anthracene 2000-2400 Rhod amine B 5100-5900 Tetraphenylprophine 4100-4250 Pyrene 2000-3800 9,10 dichlor oanthracene 2000-4500 Diethyl pseudo-isocyanine iodide 5000-6000 Beta-cardene 3000-5000 Selenium 3000-5000 Bis(p-aminophenyl)1,3,4 triazole derivatives 3600-4200 Aromatc azomethines 3600-4000 Pyrazoline derivatives 3600-4200 The weakly absorbed actinic radiation which penetrates to the middle and touter layers of the photoconductor tends t ?alt he e :layers to become temporarily semiconductive, thereby decreasing the ability thereof to accept and retain the surface electrostatic charge required for image formation. The partially transmitted actinic radiation acts to decrease image density and contrast in a manner which is undesirable and acoordingly must be avoided. Therefore, pre-illumination from the front or charged side of the photoconductor, while mechanically simple to obtain, is considered ineffective since it would seriously degrade image density .and contrast.

According to our invention, the back illumination or pre-illumination is preferably applied just prior to, but not simultaneously with the charging step. We have found that the difference in time between the pre-illumination of the photoconductor and the charging step should be limited to the period of time 'of darkness that prevails during which the particular photoconductor used can retain an appreciable fraction of its photoinduced semiconductivity, i.e., the effective duration of its memory from the pre-illumination. Thus, depending on the preillumination of the photoconducting layer, the charge carrier mobility, the electron trap density, impurities present, and other electronic parameters of the particular photoconductor used, memory periods vary from fraetions of a second to many minutes or even hours. 'For example, in the case of a photoconductive layer of Zinc oxide dispersed in insulating :binder resins, it is permissible to allow a period as long as two minutes between the preillumination and the charging steps Without serious loss of the desired eiTect. When utilizing photoconductors of a short memory, a mechanical transport device of adequate speed must be provided in order to assure sufliciently fast movement of the photoconductor between the pre-illumination and the changing steps of the electrophotographic printer.

The minimum actinic exposure required to render the back layer of the photoconductor sufiicient-ly conductive for the purposes of the present invention is about equal to the exposure required to form an electrostatic charge latent image on the front surface, corrected for the opacty of the base material, provided the same wavelength range is used. However, substantially greater exposures, that is, up to 20 times minimum, are not detrimental.

Accordingly, the upper limit of the period between the pre-illumination of the photoconductor and the charging steps is set by the memory period of the photoconductor. However, overlap between the pre-illumination and the charging steps should be minimized in order to provide optimum charge density :and image contrast on the photoconductor. Image density and contrast are deleteriously affected if illumination is continued after charging has ceased.

The physical effect obtained in accordance with the present invention is believed to be the formation of a photo-induced, temporarily semi-conductive layer on the back side of the photoconductive insulating member.

Referrin g now to FIGURES 1, 2 and 3, there is shown a transl ucent hacking material 11 supporting a photoconductive insulating layer 112. The arrows 13 represent the illumination directed onto the hacking 11, which being absor-bed, causes the .formation of a temporarily semiconductive strip 14 on the back surface of the photoconductive insulating layer 12.

By subsequently charging the surface 15 of the layer 12, "image charges" of opposite polarity are formed in the strip 14, whereby charge carrie rs, indicated by plus signs, migrate to positions exactly opposite to the surface charges indicated by minus signs, on the front surface 15. Thus, a uniform, dense dipole area is formed, which is suitable for the subsequent *for-mation of electrostatic images.

Re ferring now to FIGURE 4, there is shown by way of example, a simple electrophotographic apparatus consisting of conventional Components, except `for the addition of the pre-illuminating unit so that this app-aratus may operate in accordance with the herein described invention.

A continuous Web of electrophotographc material 20 taken from the supply roll 21 passes through the various stages of the apparatus, and is wound up on the finished product roll 22. In this apparatus the phot-oconductive coa-ting -of the web faoes downwardly. The support carrying the photoconductive coating is preferably filmbase material, so as to permit light to pass through to the back surface of the photoconductive coating. However, any material not opaque to actinic light may serve as such a support.

The web 20 may be wound through by any suitable means not shown here, either at a continuo us rate, or in step-by-step mode of operation, depending on the purpose and copying requirement of the apparatus.

As the web 20 is unrolled from the supply roll 21 it passes a contact finger 23, which is electrically at ground potential, and engages the edge of the web 20, the-reby Contacting the photoconductive layer. The purpose of the contact finger 23 is to form a "return circuit to ground fo!? the corona charge -unit 24. The latter may consist of several fine wires 25 running transverse to the web, and con,- nected to a suitable direct circuit power supply as indicated by a block diagram.

The pre-illuminating unit 26 may have various forms depending upon the type of illu mination desired, and the intensity requirement. `By way of example, the unit shown here comprises a housing or reflector 27 surrounding a pair of fluorescent tubula-r light sources 28 and 28'. The 'location of the pre-illuminating unit 26 is so -chosen as to be ahead of the charging unit 24, since as explained hereinbefore, pre-illurnination is effected preferably prior to the blanket charging of the receptive surface of the photoconductive material. When the web 20' passes the preilluminating -unit 26 the photoconductive coating thereof is conditioned to for-m a thin temporary semi-conductive layer facing the insulating support. Upon anriving at the c harging point above the unit 24, the 'face side of the photoconductor receives a blanket charge, the temporarily semi-conductive layer at the back acting as a conductive base. The web is now ready for imagewise dischar ging of the `dipoles so formed, and this is accomplished 'by a suitable projecting unit shown here schematically, by a lens 30 which projects a reflected image from the original 3-1 when the latter is illuminated by the lamps 32 and 33. After suflicient exposure has taken place, the web is passed t-o the developing station 35, which by way of llustration, is shown hereto be of the liquid type, consisting of a container 36 filled with insulating liquid 37 in which is Suspended suitable electrostatically charged ton'er particles. Guidng rollers 38 and 39 pass the web around the applicator roller 40 which is immersed in the s ol-uti on 37. Similar guiding rollers 41 and 42 pass the web toward the finished product roll 22. The image is now developed by the adhe-rence of the toner partieles and is fixed -by the heating unit 43 containing suitable electrical heaters 44 and 45.

What is cl-aimed is:

1. In a process of producing an electrostatic latent image on electrophotographic material of the type consisting of a p'hotoconductive layer on a translucent insulating support wherein are employed the steps of applyng a uniform electrostatic charge to a receptive surface o f the ph-otoconductive layer -of said material, of exposng said layer to a light-image pattern, and of finally developing and fiXing said image; the improvement which comprises pre-illuminating the surface opposite said receptive surface of said photoeonductive layer prior to the application of said charge, to the extent of producing a semicon'ductve l-ayer at said opposite surface, said last mentioned layer forming efectively a conductive base.

2. The process according to claim 1 wherein the photoconductive layer of said material is composed of zinc oXide.

3. The process according to claim 1 wherein the photoconductive layer of said material comprises a transparent Organic photoconductor.

4. A process according to claim 1 wherein said material comprises a self-supporting clear film of Organic photoconductive substance.

5. A process according to claim 1 wherein said material consists of vitreous selenium.

6. In :a process of `forming an electrostatic latent image on a Xerographic plate 'having a .translucent hacking of poor conductivty, wherein are empl oyed the steps of applying a uniform electrostatic surface charge to the photoconductor of said plate, of exposing sai'd conductor to a light-image pattern, and of finally developing and transferring said image ont-o a receiving sheet; the improvement which comprises pre-illuminating the rear surface of said photoconductor -overlying said translucent 'hacking prior to the application of said surface charge, Whereby said photoconductor is corditioned to have a semi-conductive layer at said rear surface, said last -mentioned layer forming effectively a conductive base.

7. The process in :accordance with claim 1 wherein the pre-illuminating step is effected with actinic light of a wavelength falling within the region of intense absorption of said photoconductive material.

8. The process in aocordance with claim 1 wherein the pre-illuminating step is effected with actinic light of such wavelength duration and intensity as to have no more than 10% of the radiation penetrating beyond about '10% of the thickness of said photoeonductive substance.

References Cited by the Examier UNITED STATES PATENTS 2,937,943 5/ 1960 Walk-up 96 1 2,955,9*38 1011960 Steinhilper 96-1 2,963;36-5 1'2/ 1960 Grieg 96-1 2,979,402 4/ 1961 Greig 96-1 3,037,86-1 6/1962 Hoegl et al. 96-1 3,04I,16 7 6/1962 Blakney 96-1 3,121,007 2/ 1964 Middleton et a-l. 96-1 OTHER REFERENCES :Photo Dictionary and Quick Reference Guide, Morgan and Morgan, Inc., New York, 1957, page 39.

RCA Review, December 1959, pages 753-769.

Helvetica Physica Acta, 30, 1957, pages 495-503.

NORMAN G. TORCHIN, Primary Exam'ner.

G. H. BJORGE, A. LIBERMAN, C. E. VAN HORN,

Assistant Exam'ners. 

1. IN A PROCESS OF PRODUCING AN ELECTROSTATIC LATENT IMAGE ON ELECTROPHOTOGRAPHIC MATERIAL OF THE TYPE CONSISTING OF A PHOTOCONDUCTIVE LAYER ON A TRANSLUCENT INSULATING SUPPORT WHEREIN ARE EMPLOYED THE STEPS OF APPLYING A UNIFORM ELECTROSTATIC CHARGE TO A RECEPTIVE SURFACE OF THE PHOTOCONDUCTIVE LAYER OF SAID MATERIAL, OF EXPOSING SAID LAYER TO A LIGHT-IMAGE PATTERN, AND OF FINALLY DEVELOPING AND FIXING SAID IMAGE; THE IMPROVEMENT WHICH COM- 